The Master

This incarnation of the Master travelled with the Ninth Doctor and had an android body.

The circumstances behind the creation of this incarnation of the Master remain undisclosed, however, he noted that he was less than happy with this arrangement, saying that it was "an offer I was foolish enough to accept". He also said that he "was of aid to the Doctor during the events that damaged him". This implies that the Doctor helped create him. His android body was limited to functioning within the Doctor's TARDIS. Therefore he could not leave it and, in effect, he had become the Doctor's companion.

_{Biography from the TARDIS Data Core article, licensed under CC-BY-SA}